It is known in prior art to provide an SCR filter made up of a filter and an SCR catalyst (selective catalytic reduction NOx catalyst) supported on the filter in an exhaust passage of an internal combustion engine. The SCR filter has a capability of reducing NOx in the exhaust gas by using ammonia as reducing agent. The filter has the function of trapping particulate matter (which will be hereinafter referred to as “PM”) in the exhaust gas.
PTL 1 discloses a technology used in an exhaust gas purification system including an SCR catalyst provided downstream of a filter in an exhaust passage of an internal combustion engine to correct the quantity of reducing agent to be supplied to the SCR catalyst taking account of the amount of PM deposited in the filter. In the technology disclosed in PTL 1, the quantity of reducing agent supplied to the SCR filter is to be corrected because NO2 in the exhaust gas is reduced to NO by the PM deposited in the filter and the proper quantity of reducing agent to be supplied to the SCR catalyst for removal of NOx changes accordingly.
NON-PTL 1 teaches that increases in the amount of PM deposited in an SCR filter make the ammonia adsorption amount defined as the amount of ammonia adsorbed in an SCR catalyst supported on the SCR filter more apt to increase.